The faces of the rolls in a paper/board machine tend to be coated with impurities coming from the process and with material of the doctor blade. For removal of these materials from the roll faces, doctor blades are employed.
Typical commercial blades are described, for example, in the U.S. Pat. No. 4,549,933 and in the U.S. Pat. No. 1,694,755.
In U.S. Pat. No. 4,549,933, a doctor blade for a paper machine is described, which consists of a number of alternating layers of fiber and carbon fiber so that the fiber layer may be composed of cotton, paper, fiberglass, or equivalent. Such a blade detaches contaminations, but, on the other, foreign material is separated from the blade onto the roll face.
In the U.S. Pat. No. 1,694,755, a doctor blade meant for cleaning of rolls and cylinders is described for use in pulp and paper industry. This blade is made of a composite material, in which, over the thickness of the blade, layers of fiber and fabric filler perpendicular to the blade edge alternate. This doctor blade is used for cleaning of the roll faces. Besides the fabric filler, for the doctor, carbon band, boron fiber or tungsten is used as fiber, and the alignment of the fibers is perpendicular to the direction of the blade.
None of said blade constructions contains particles of abrasive material.
In U.S. Pat. No. 5,174,862, a polishing doctor blade is described for polishing the metal face of a calender roll. The machining face placed at the head of the blade. comprises grinding particles mixed with an epoxy matrix, such as carbides or diamonds. The blade does not operate as a doctoring blade. It is meant exclusively for calender rolls, at which its function is exclusively to grind/polish the face of the calender roll.
In the FI Patent Application No. 941620, a method and an equipment are described for conditioning of the coating on a roll in a paper machine. What is concerned is not a doctor blade, but in the method of FI-941620 the roll is ground by means of a separate grinding rib to be attached to a doctor blade so as to correct the roughness of the roll face to make the face either smoother or rougher, as required. In such a case, the conditioning of the roll face takes place during a standstill on-site, i.e without removing the roll from the machine, but the device does not operate during production or at a production speed.
Coating with a foreign material often results in alteration of the surface properties of a roll, which again deteriorates the runnability and the properties of paper. If the coating is intensive, the roll face may become excessively smooth, which results, on a center roll in the press, for example, in more difficult separation of the paper web, passing of the web through the doctor, and, thus, in increased susceptibility of web breaks. Besides the coating effect, a second negative aspect in the case of metal blades is scratching of the roll face or, in some cases, penetration of the blade into the roll coating if a polymer-based coating is concerned. Also, excessive roughening of a roll face, which occurs with prolonged operation of metal blades, causes. deterioration of the operating capacity of the roll face.
Smoothing or roughening of a roll face is typical of a roll that contains ceramic coating material, but it also occurs with other coatings. The coating effect of the blades of reinforced plastic commonly employed with ceramic roll coatings mainly results from adhesion of particles detached from the blade by abrasion or from melting of the matrix plastic onto the roll face. On the other hand, materials coming from the process also adhere to the roll face, which materials cannot be removed by the doctor from the recesses in the roughness of the face, as the doctor cannot remove the layer of impurities deposited on these materials “anchored” on the bottom of said recesses either, because the adhesion is too strong.
Even the best conventional doctor blade cannot remove all the undesirable material already adhering to the roll face from the roll face, but the blade often brings more material to the roll face. This is why there has been a need to develop a doctor blade which removes undesirable contaminations or other material coated onto the roll face from the roll face to a greater extent than the blade itself brings onto the roll face and which doctor blade keeps the roll face in its original condition or restores it to its original condition.